Crocodilia (or Crocodylia, both /krɒkəˈdɪliə/) is an order of mostly
large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years
ago in the Late Cretaceous period (Cenomanian stage) and are the closest living relatives of birds,
as the two groups are the only known survivors of the Archosauria. Members of the order's total
group, the clade Pseudosuchia, appeared about 250 million years ago in the Early Triassic period,
and diversified during the Mesozoic era. The order Crocodilia includes the true
crocodiles (family Crocodylidae), the alligators and caimans (family Alligatoridae), and
the gharial and false gharial (family Gavialidae). Although the term 'crocodiles' is sometimes used to
refer to all of these, crocodilians is a less ambiguous vernacular term for members of this group.
Large, solidly built, lizard-like reptiles, crocodilians have long flattened snouts, laterally compressed
tails, and eyes, ears, and nostrils at the top of the head. They swim well and can move on land in a
"high walk" and a "low walk", while smaller species are even capable of galloping. Their skin is thick
and covered in non-overlapping scales. They have conical, peg-like teeth and a powerful bite. They
have a four-chambered heart and, somewhat like birds, a unidirectional looping system of airflow
within the lungs, but like other living reptiles they are ectotherms.
Crocodilians are found mainly in lowlands in the tropics, but alligators also live in the southeastern
United States and the Yangtze River in China. They are largely carnivorous, the various species
feeding on animals such as fish, crustaceans, molluscs, birds, and mammals; some species like the
Indian gharial are specialised feeders, while others like the saltwater crocodile have generalised
diets. Crocodilians are typically solitary and territorial, though cooperative feeding does occur.
During breeding, dominant males try to monopolise available females. Females lay eggs in holes or
in mounds and, unlike most other reptiles, care for their hatched young.
Some species of crocodilians are known to have attacked humans. The largest number of attacks
comes from the Nile crocodile. Humans are the greatest threat to crocodilian populations through
activities that include hunting, poaching, and habitat destruction, but farming of crocodilians has
greatly reduced unlawful trading in wild skins. Artistic and literary representations of crocodilians
have appeared in human cultures around the world since Ancient Egypt. The earliest known mention
of the story that crocodiles weep for their victims was in the 9th century; it was later spread by
Sir John Mandeville in 1400 and then by William Shakespeare in the late 16th century and early 17th
century.
Crocodilia and Crocodylia have been used interchangeably for decades starting with Schmidt's
redescription of the group from the formerly defunct term Loricata.[1] Schmidt used the older term
Crocodilia, based on Owen's original name for the group.[2] Shortly after, Wermuth opted for
Crocodylia as the proper name for this redescribed group,[3] basing it on the type
genus Crocodylus (Laurenti, 1768).[4] Dundee—in a revision of many reptilian and amphibian
names—argued strongly for Crocodylia to be the spelling for the group.[5] However, it was not until
the advent of cladistics and phylogenetic nomenclature that a more solid justification for assuming
one spelling over the other was proposed.[6]
Prior to 1988, Crocodilia/Crocodylia was a group that encompassed the modern-day animals as well
as their more distant relatives now in the larger groups
called Crocodylomorpha and Pseudosuchia.[6] Under its current definition as a crown group (as
opposed to a stem-based group), Crocodylia is now restricted to only the last common ancestor of
today's modern-day crocodilians (alligators, crocodiles, and gharials) and all of its descendants
(living or extinct).[6] This distinction is more important for paleontologists studying crocodilian
evolution. As such, the alternate spellings Crocodilia and Crocodylia are still used interchangeably in
the neontological literature.
Crocodilia[2] appears to be a Latinizing of the Greek κροκόδειλος (crocodeilos), which means
both lizard and Nile crocodile.[7] Crocodylia, as coined by Wermuth,[3] in regards to the
genus Crocodylus appears to be derived from the ancient Greek[8] κρόκη (kroke)—meaning shingle
or pebble—and δρîλος or δρεîλος (dr(e)ilos) for "worm". The name may refer to the animal's habit of
basking on the pebbled shores of the Nile.[9]
Crocodilians range in size from the Paleosuchus and Osteolaemus species, which reach 1–1.5 m
(3 ft 3 in – 4 ft 11 in), to the saltwater crocodile, which reaches 7 m (23 ft) and weighs up to 2,000 kg
(4,400 lb), though some prehistoric species such as the late Cretaceous Deinosuchus were even
larger at up to about 11 m (36 ft)[10] and 3,450 kg (7,610 lb).[11] They tend to be sexually dimorphic,
with males much larger than females.[12] Though there is diversity in snout and tooth shape, all
crocodilian species have essentially the same body morphology.[11] They have solidly built, lizard-like
bodies with elongated, flattened snouts and laterally compressed tails.[12] Their limbs are reduced in
size; the front feet have five digits with little or no webbing, and the hind feet have four webbed digits
and a rudimentary fifth.[13] The skeleton is somewhat typical of tetrapods, although the skull, pelvis
and ribs are specialised;[12] in particular, the cartilaginous processes of the ribs allow the thorax to
collapse during diving and the structure of the pelvis can accommodate large masses of food,[14] or
more air in the lungs.[15] Both sexes have a cloaca, a single chamber and outlet at the base of the tail
into which the intestinal, urinary and genital tracts open.[12] It houses the penis in males and
the clitoris in females.[16] The crocodilian penis is permanently erect and relies on cloacal muscles for
eversion and elastic ligaments and a tendon for recoil.[17] The gonads are located near the kidneys.[18]
The eyes, ears and nostrils of crocodilians are at the top of the head. This allows them to stalk their
prey with most of their bodies underwater.[19] Crocodilians possess a tapetum lucidum which
enhances vision in low light.[13] While eyesight is fairly good in air, it is significantly weakened
underwater.[20] The fovea in other vertebrates is usually circular, but in crocodiles it is a horizontal bar
of tightly packed receptors across the middle of the retina. When the animal completely submerges,
the nictitating membranes cover its eyes. In addition, glands on the nictitating membrane secrete a
salty lubricant that keeps the eye clean. When a crocodilian leaves the water and dries off, this
substance is visible as "tears".[13]
The ears are adapted for hearing both in air and underwater, and the eardrums are protected by
flaps that can be opened or closed by muscles.[21] Crocodilians have a wide hearing range, with
sensitivity comparable to most birds and many mammals.[22] They have only one olfactory
chamber and the vomeronasal organ is absent in the adults[23] indicating all olfactory perception is
limited to the olfactory system. Behavioural and olfactometer experiments indicate that crocodiles
detect both air-borne and water-soluble chemicals and use their olfactory system for hunting. When
above water, crocodiles enhance their ability to detect volatile odorants by gular pumping, a rhythmic
movement of the floor of the pharynx.[24][25] The well-developed trigeminal nerve allows them to detect
vibrations in the water (such as those made by potential prey).[26] The tongue cannot move freely but
is held in place by a folded membrane.[14] They appear to have lost their pineal organ, but still show
signs of melatonin rhythms.[27] The crocodilian larynx is capable of complex motor control similar to
birds and mammals.[28] While the brain of a crocodilian is fairly small, it is capable of greater learning
than most reptiles.[29]
Crocodilians are excellent swimmers. During aquatic locomotion, the muscular tail undulates from
side to side to drive the animal through the water while the limbs are held close to the body to
reduce drag.[19][30] When the animal needs to stop, steer, or manoeuvre in a different direction, the
limbs are splayed out.[19] Crocodilians generally cruise slowly on the surface or underwater with
gentle sinuous movements of the tail, but when pursued or when chasing prey they can move
rapidly.[31] Crocodilians are less well-adapted for moving on land, and are unusual among vertebrates
in having two different means of terrestrial locomotion: the "high walk" and the "low walk".[13] Their
ankle joints flex in a different way from those of other reptiles, a feature they share with some early
archosaurs. One of the upper row of ankle bones, the astragalus, moves with the tibia and fibula.
The other, the calcaneum, is functionally part of the foot, and has a socket into which a peg from the
astragalus fits. The result is that the legs can be held almost vertically beneath the body when on
land, and the foot can swivel during